perf: Use split_at in split

[ritchie46]

This will save a lot of double null_counts.

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.41%. Comparing base (13d68ae) to head (9a05fc7).

Files	Patch %	Lines
crates/polars-core/src/utils/mod.rs	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16865      +/-   ##
==========================================
+ Coverage   81.36%   81.41%   +0.05%     
==========================================
  Files        1425     1425              
  Lines      187910   187926      +16     
  Branches     2705     2705              
==========================================
+ Hits       152885   152995     +110     
+ Misses      34529    34435      -94     
  Partials      496      496              

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